Energy harvesting unit can achieve energy harvest and self-supplying for wireless sensors of smart substations from electromagnetic environment. It will have wide application prospect in the smart grid. Some main constraints and corresponding solutions of the conditioning topology of capacitive energy harvesting device are analyzed in details. A topology design of passive device of the conditioning circuitry is proposed， and modeling methodology is also established accordingly. Both large-signal model and dynamic small disturbance model of the topology are used to analyze static and dynamic characteristics of the conditioning circuitry. Furthermore， based on theoretical deduction under the critical operation condition， a multiple parameter optimization method is proposed. Simulations are completed with PSPICE software to verify effectiveness of the proposed modeling scheme and parameter optimization methodology.